(define (string-merge-iter str-list-1 str-list-2)
 (merge-iter-helper  str-list-1 str-list-2 ""))

(define (merge-iter-helper str-list-1 str-list-2 res)
 (cond ((null? str-list-1) 
          (string-append res (apply string-append str-list-2)))
       ((null? str-list-2) 
          (string-append res (apply string-append str-list-1)))
       (else (merge-iter-helper
                (cdr str-list-1) 
                (cdr str-list-2)
                (string-append 
                    res (car str-list-1) (car str-list-2))))))